Method of treating steel



United States Patent like 7 3,039,902 Patented June 19, 1962 3,039,902 METHOD OF TREATING STEEL Clarence L. Miller, Jr., Pittsburgh, and Paul E. Perry, Tarentum, Pa., assignors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Filed Apr. 15, 1958, Ser. No. 728,557 8 Claims. (Cl. 148113) This invention relates to a method of reducing the sulfur content of flat rolled steel products and relates in particular to improvements in the treatment of electrical grades of silicon steel strip.

In the processing and manufacture of steel products and particularly electrical grades of steel, it is frequently desirable to lower the sulfur content to a figure below that present when the metal is in a molten state. For example, in silicon grades of electrical steel used in magnetic cores for power and distribution transformers etc., it is common practice to reduce the steel to strip and sheet form in a manner designed to impart improved electrical properties in the direction of rolling. High directional permeability and low core loss are obtained in these steels by alternate cold rolling and heat treatment of the hot rolled mill product. In this manner [100] (110) grain texture or the crystallographic grain orientation that provides improved electrical properties is obtained. After the last cold rolling, the steel is in a condition that exhibits poor magnetic properties; however, after a final heat treatment within the temperature range of 1600 F. to 2250 F., the desired directional crystallographic orientation occurs. Such a treatment is taught in US. Patent No. 1,965,559 to Goss.

It is common practice to box anneal coils of steel strip as a final heat treatment Within the temperature of from about 2000 F. to 2250 F. in a substantially non-oxidizing or reducing atmosphere. This heat treatment not only effects crystallographic orientation while the coil is coming to temperature but also effects a reduction of the sulfur content which migrates to the surface of the steel during the heat treatment. The elimination of sulfur is important at this stage in obtaining the best magnetic properties in that the continuing presence of sulfur tends to promote internal stresses which in turn result in lower permeability at low levels of induction.

The high temperature treatment, to be effective in eliminating sulfur must be continued for a long period of time, sometimes exceeding 50 hours. To heat treat material at such a high temperature for along period of time is expensive and wasteful not only in the energy expended in obtaining such a temperature and maintaining it for such a length of time but also in the damaging efiects on furnace structure such as erosion of furnace liners and oxidation of the heating elements. The requirement for such a long time treatment at high temperatures renders any method of reducing the time of heat treatment required for the sulfur removal a significant advance in the production of electrical grades of silicon steel.

It has now been found that the heat treatment time required to reduce the sulfur content of flat rolled silicon grades of steel containing from 1 to 4% silicon may be materially reduced by bringing such steel products into contact with certain chemical compounds during heat treatment. In the process of the present invention silicon grade electrical steels that contain .020% sulfur may be heat treated within the temperature range of from 2000 F. to 2250 F. for less than an hour to reduce the sulfur content to from about 006% to 012%.

It is, therefore, the object of the present invention to provide a method of reducing the time at temperature heat treatment required to lower the sulfur content of steel.

It is a further object of the present invention to bring at least one alkali metal salt or alkali earth metal salt selected from the group consisting of calcium carbon-ate, lithium carbonate, sodium nitrate and potassium nitrate into contact with electrical grades of steel during heat treatment at temperatures ranging from about 2000 F. to 2250 F.

It is also the object of the present invention to coat electrical grades of steel with at least one material selected from the group calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate and heat treat at temperatures of from 2000 F. to 2250 F. in order to reduce the sulfur content thereof.

Other objects and advantageous features of the present invention will be obvious from the following description.

In general, the present invention relates to a method of reducing the sulfur content of steel by bringing the steel into contact with at least one alkali metal compound or alkali earth metal compound of the class calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate and heat treating the steel within a temperature range of from 2000 F. to 2250 F. The invention relates in particular to coating silicon grades of electrical steels that contain from 1% to 4% silicon and that have been processed in such a manner that subsequent heat treatment will obtain (100) grain texture with at least one material selected from the group calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate and heat treating said steels within the temperature range of from 2000 F. to 2250 F. in substantially nonoxidizing or reducing atmosphere. Another preferred embodiment of the present invention is to provide a coating formed of magnesium hydroxide combined with material selected from the above recited sulfur removing materials on the surfaces of electrical grades of silicon steel strip, prior to final annealing. Also, it is preferable in carrying out the present invention to provide coatings of the present materials by immersing steel strip or sheet in an aqueous bath that contains the desired materials.

After final cold rolling of silicon steel strip processed for preferred magnetic orientation, the strip is usually at a gauge of from about .010 inch to .018 inch. Desired crystallographic orientation does not exist at this stage and the electrical properties are low; however, subsequent heat treatment at a temperature exceeding about 1600 F. for a relatively longer period of time effects the desired crystallographic orientation. Generally a heat treatment of from about 1600 F. to 1800 F. for a period of 9 or 10 hours is suflicient to effect substantially complete [100] orientation. However, the best electrical or magnetic properties are not obtained by subjecting the material to such treatment because of the relatively high sulfur content, usually around .020%, and small residual internal stresses. Such a sulfur content is inevitable due tofurnace melting practices and sulfur pickup in subsequent treatment. Also, the sulfur is believed to enhance the obtaining of the desired potential directional magnetic properties and therefore a sulfur content of about .020% is preferred prior to the final heat treatment but at this time must be reduced to obtain optimum results. Conventional practice is to box anneal coils of the final gage strip material, in a non-oxidizing or reducing atmosphere at a furnace temperature of from 2000 F. to 2250 F. The steel must be maintained at the high temperature for a long enough period of time to permit the sulfur content to be removed at the surface.

in the method of the present invention, it has been found that the time at temperature may be materially reduced if the steel strip material is contacted with a material from the group calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate. The time required to reduce the sulfur content of the silicon grades of steel containing about .020% sulfur to the desired level of approximately .006% to .012% sulfur is decreased to less than 1 hour. Thus, it can be seen that substantial savings are effected by the practice of the present invention. Also, heat treatments of less than one hour make it possible to conduct such treatment on steel strip in a continuous manner, such as strand anneal, instead of the usual time consuming cumbersome box anneal.

Common coatings now in use on electrical grades of silicon steel strip involve the use of magnesium compounds and particularly magnesium oxide. Such coatings when subjected to heat treatment effect a magnesium-ironsilicon base coating that is ideal for subsequent phosphate coatings employed as protective and insulating coatings on electrical grades of silicon steel. A method of applying such a phosphate coating is taught in US. Patent 2,753,282. The magnesium-iron-silicon base coating is generally obtained by passing the strip material through an aqueous bath that contains a dispersion or suspension of magnesium hydroxide. The strip is then box annealed in a non-oxidizing, usually reducing, atmosphere at about 2000 F. to 2250 F.

In applying the materials of the present invention to electrical grades of silicon steel strip it has been found to be highly advantageous and preferable, where the steel is to be subsequently phosphate coated, to combine the sulfur removing materials with the magnesium hydroxide dispersion. In this manner the desired base for subsequent phosphate coating is obtained simultaneously while effecting accelerated sulfur removal. The magnesium hydroxide appears to enhance the sulfur removing ability of the materials of the present invention. When the combined magnesium hydroxide-sulfur removing materials are employed it has been found that coatings of .07 ounces per square foot of strip surface Where the ratio of Mg(OH) to sulfur removing material is about 10 to 1 has accelerated sulfur removal during the 2000 F. to 2200 F. box anneal. It is preferred, particularly where subsequent phosphate coatings are to be applied, the Mg(OH) should exceed the amount of sulfur removing materials of the present invention.

A continuous coating of one or more of the alkali metal or alkali earth metal compounds selected from the group calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate of any weight, with or without combined magnesium hydroxide will accelerate sulfur removal during the high temperature heat treatment. An effective weight of application is from .002 to 1.5 ounces per square foot of strip surface area. Where these materials are codeposited from an aqueous solution containing magnesium hydroxide the lower portion of the range of about .002 to .007 ounce per square foot is the more effective. Thus, where an aqueous suspension containing 10 parts Mg(OH) to one part of the present materials and 89 parts water is employed the optimum weight of application of the combined materials is .02 to .07 ounce per square foot. Where the present materials are deposited independently of Mg(OI-I) a total deposit of .02 to .07 ounce per square foot produces optimum results. When the combined or noncombined (with Mg(OH) coatings exceed about .07 ounce per square foot of application the effectiveness of accelerated sulfur removal is reduced, however, some benefits may be experienced in applications up to 1.5 ounces per square foot. Where the combined coatings or individual coatings (without Mg(OH) are less than .02 ounce per square foot of applications the advantages of such a coating in accelerating sulfur removal are greatly diminished.

Although the use of aqueous suspensions is aneconomical and convenient method of applying the materials of the present invention, and coatings, as such, are preferred due to the more intimate contact between the chemicals involved and the strip surface, it is to be understood the sulfur reducing materials may be brought into contact with the surface of the strip in any convenient manner. Such material may be dusted onto the surface of the strip or may be brought into contact with the surface of the strip or may be projected into the box annealing furnace with the coil of strip material prior to the box annealing heat treatment. However, optimum results are obtained with the coated material. Also, the coated material may be continuously strand annealed in some instances rather than box annealed.

' Where coatings are applied to steel strip by continuously immersing the strip in aqueous suspensions of the materials of the present invention, the strength of the bath or aqueous suspension is not critical in that the thickness of the coating obtained in such a manner depends upon the speed it passes through the suspension as well as the amount of material suspended. Excellent results have been obtained by employing baths containing as little as .5 by weight, and as much as 5%, by weight, of the suspended compounds. Such suspensions have been successfully employed in coating strip where magnesium hydroxide is co-deposited in a ratio of 10 parts magnesium hydroxide and one part of the material of the present invention and where the present materials are singularly employed.

The high temperature heat treatment of the coated steel products must, as in the case of conventional practice, be carried out in a substantially non-oxidizing or reducing atmosphere. Inert gases such as argon or substantial vacuums may be conveniently employed; however, a reducing atmosphere, such as is provided by hydrogen gas, is sometimes preferred.

The following specific examples are given to illustrate the method of the present invention and in no way limits the invention to the exact procedures set forth.

Electrical grades of silicon steel (SX10) were hot rolled and alternately cold rolled and heat treated to impart desired potential magnetic properties. The final gauge was approximately .014". A representative composition of the SX1 0 steel is as follows:

C .025 Cr .010 Mn .010 Ni .10 P .020 Cu .075 S .020 A1 .010 Si 3.00 Sn .010

Samples cut from each of the lots tested were coated with the sulfur removing materials of the present invention by dipping them into water suspensions of the desired materials. The water suspensions contained approximately 10% by weight of Mg(OH) and 1% by weight of the various coating materials. The samples were then annealed at about 2200 F. for 30 minutes in an argon atmosphere. Sulfur content was determmed both before and after testing. Results shown 1n Table I below.

Table I Percent S Lot Coating Before Heat After Heat Treatment Treatment 53-5676 None .020 .017 .020 .016 .023 .017 .021 .015 .020 .005 .020 .006 .023 .007 .021 .005 .020 .006 .020 .007 .023 .006 .021 .006 .020 .007 .020 .005 .023 .006 .021 .004 .020 .007 .020 .008 .023 .008 .021 .007

Samples of SX-lO steel, such as described above were also immersed in aqueous suspensions of Mg(OH) and 5% by weight Li CO and 5% by Weight Li CO (no Mg(OH) present). As above these samples were heated to 2200 F. for 30 minutes in an argon atmosphere. Results are shown in Table II.

Table II Percent S Lot Coating Before After Heat Heat Tree tm ant Treatment 53-5676 LizCOz-l-hig (OH);. .020 006 514239.. LigOOz+hIg(OH)2 020 .006 51-8923 Li2CO +hIg(OI-I)z. 023 005 51-5259 LizCOa-i-hIg(OH)- 021 005 53-5676 i200; 020 011 51-4239- Li2CO: 020 008 51-8923- Li2COs-- .023 009 51-5259 LizCOa O21 006 Additional tests were conducted on samples employing the various coating materials while varying the time at temperature. The temperature and atmosphere (2200 F.argon) were the same as employed in the tests reported in Tables I and II. The tests reported in Table III below represents average sulfur contents of at least two tests on SX1O steel. The sulfur content before testing was .020% greater.

Additional tests were conducted on the SX10 steel while employing calcium carbonate as the sole coating. Otherwise test conditions were similar to the above (2200 F.30 minutes-argon atmosphere). Results of these tests are given in Table IV.

Table IV Weight of Percent S coating, Lot ounces per sq. it. Before Heat After Heat Treatment Treatment Additional tests were conducted on SX-10 samples while employing various of the coating materials combined with Mg(OH) The samples exhibited various thicknesses of coatings and contained 1 part of the coating materials of the present invention to 10 parts of magnesium hydroxide. The tests were conducted in an argon atmosphere at 2200 F. for 30 minutes. Results are shown in Table V.

As can be seen by the above illustrations, by employing the coatings of the present invention the sulfur content of silicon steel may be reduced significantly within a short period of time as compared to the uncoated mate rial. For example, in Table I it is shown that where no coatings are employed, sulfur content is reduced from about .020% to at best .015% when held at 2200 F. for /2 hour in a non-oxidizing atmosphere but coated samples of 10 parts Mg(OI-I) to 1 part of the coating materials of the present invention show a sulfur content after heat treatment as low as .004%. In Table II it is shown that the coatings of the present invention may be combined with Mg(O'I-I) or may be used alone to effect a lowering of the sulfur content. In Table III it is clearly shown that the beneficial effects of the coatings of the present invention are substantially completed during the first /2 hour of the heat treatment. Two hours of heat treatment effected only a slight additional reduction of the sulfur content. In Tables IV and V it is shown that the coatings of the present invention are efiective in lowering sulfur content although extremely thin coats are applied. In Table V it is shown that .0017 ounce per square foot of Li CO combined with Mg(OH) effects a reduction in sulfur.

We claim:

1. In the process of making magnetic materials wherein hot fiat rolled silicon grades of electrical steel that contain from 1% to 4% silicon are alternately cold rolled and heat treated to obtain cube on edge crystal orientation parallel to the direction of rolling and which are held at elevated temperature in a substantially non-oxidizing atmosphere to lower the sulfur content of said steel, the improvement in combination therewith of accelerating the lowering of said sulfur content by substantially uniformly coating said cold rolled steel with from about .02 ounce per square foot to about .07 ounce per square foot of at least one material selected from the group consisting of calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate and subjecting said coated cold rolled steel to a temperature of from 2000 F. to 2250 F. in a substantially non-oxidizing atmosphere for an efiective time of up to but not including one hour.

2. In the process of making magnetic materials wherein hot flat rolled silicon grades of electrical steel that contain from 1% to 4% silicon are alternately cold rolled and heat treated to obtain [100] (110) cube on edge crystal orientation parallel to the direction of rolling and which are held at elevated temperature in a substantially nonoxidizing atmosphere to lower the sulfur content of said steel, the improvement in combination therewith of accelerating the lowering of said sulfur content by substantially uniformly coating said cold rolled steel with from about .02 ounce per square foot to about .07 ounce per square foot of calcium carbonate and subjecting said coated cold rolled steel to a temperature of from 2000 F. to 2250 F. in a substantially non-oxidizing atmosphere for an effective time of up to but not including one hour.

3. In the process of making magnetic materials wherein hot fiat rolled silicon grades of electrical steel that contain from 1% to 4% silicon are alternately cold rolled and heat treated to obtain [100] (11-0) cube on edge crystal orientation parallel to the direction of rolling and which are held at elevated temperature in a substantially non-oxidizing atmosphere to lower the sulfur content of said steel, the improvement in combination therewith of accelerating the lowering of said sulfur content by substantially uniformly coating said cold rolled steel with from about .02 ounce per square foot to about .07 ounce per square foot of lithium carbonate and subjecting said coated cold rolled steel to a temperature of from 2000 F. to 2250 F. in a substantially non-oxidizing atmosphere for an effective time of up to but not including one hour.

4. In the process of making magnetic materials wherein hot fiat rolled silicon grades of electrical steel that contain from 1% to 4% silicon are alternately cold rolled and heat treated to obtain [100] (110) cube on edge crystal orientation parallel to the direction of rolling and which are held at elevated temperature in a substantially non-oxidizing atmosphere to lower the sulfur content of said steel, the improvement in combination therewith of accelerating the lowering of said sulfur content by substantially uniformly coating said cold rolled steel with from about .02 ounce per square foot to about .07 ounce per square foot of sodium nitrate and subjecting said coated cold rolled steel to a temperature of from 2000 F. to 2250 F. in a substantially non-oxidizing atmosphere for an effective time of up to but not including one hour.

5. In the process of making magnetic materials wherein hot flat rolled silicon grades of electrical steel that contain from 1% to 4% silicon are alternately cold rolled and heat treated to obtain [100] (110) cube on edge crystal orientation parallel to the direction of rolling and which are held at elevated temperature in a substantially non-oxidizing atmosphere to lower the sulfur content of said steel, the improvement in combination therewith of accelerating the lowering of said sulfur content by substantially uniformly coating said cold rolled steel with from about .02 ounce per square foot to about .07 ounce per square foot of potassium nitrate and subjecting said coated cold rolled steel to a temperature of from 2000 F. to 2250 F. in a substantially non-oxidizing atmosphere for an effective time of up to but not including one hour.

6. The method of reducing sulfur in flat rolled silicon grades of electrical steel that contain from 1% to 4% silicon which comprises, immersing said steel in an aqueous bath that contains a suspension of from about .5 to

5 percent of at least one material selected from the group consisting of calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate, withdrawing said steel so as to leave a substantially uniform coating of said r material of a dry weight of from about .002 ounce per square foot to about 1.5 ounces per square foot and subjecting said coated steel to a temperature of from 8 2000 F. to 2250 F. in a substantially non-oxidizing atmosphere for an effective time of up to but not including one hour.

7. The method of reducing sulfur in flat rolled silicon grades of electrical steel that contain from 1% to 4% silicon which comprises, immersing said steel in an aqueous bath that contains a suspension of from about .5 to 5 percent of magnesium hydroxide and at least one material selected from the group consisting of calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate, Withdrawing said steel so as to leave a substantially uniform dry weight coating within the range of from about .02 ounce per square foot to about .07 ounce per square foot and subjecting said coated steel to a temperature of from 2000 F. to 2250 F. in a substantially non-oxidizing atmosphere for an effective time of up to but not including one hour.

8. In the process of making magnetic materials wherein hot flat rolled silicon grades of electrical steel that contain from 1% to 4% silicon are alternately cold rolled and heat treated to obtain cube on edge crystal orientation parallel to the direction of rolling which are held at elevated temperatures in a substantially nonoxidizing atmosphere to lower the sulfur content of said steel and which are subsequently provided with an insulating phosphate coating, the improvement comprising accelerating the lowering of said sulfur content by substantially uniformly coating said cold rolled steel with from about .02 ounce per square foot to about .07 ounce per square foot of a material that consists essentially of magnesium hydroxide and that includes from .002 to .007 ounce per square foot of at least one material selected from the group consisting of calcium carbonate, lithium carbonate, sodium nitrate and potassium nitrate, and subjecting said coated cold rolled steel to a temperature of from 2000 F. to 2250 F. in a substantially nonoxidizing atmosphere for an effective time of up to but not including one hour.

References Cited in the file of this patent UNITED STATES PATENTS 2,112,084 Frey et al Mar. 22, 1938 2,204,813 Muskat June 18, 1940 2,389,497 Gat Nov. 20, 1945 2,799,575 Tisdale et al July 16, 1957 FOREIGN PATENTS 260,646 Great Britain Nov. 2, 1926 OTHER REFERENCES Journal of Metals, June 1952, pages 604-608. Bozorth: Ferromagnetism 1951, published by Van Nostrand Co., Inc., N.Y.C., page 86. 

1. IN THE PROCESS OF MAKING MAGNETIC MATERIALS WHEREIN HOT FLAT ROLLED SILICON GRADES OF ELECTRICAL STEEL THAT CONTAIN FROM 1% TO 4% SILICON ARE ALTERNATELY COLD ROLLED AND HEAT TREATED TO OBTAINED (100) (110) CUBE ON EDGE CRYSTAL ORIENTATION PARALLEL TO THE DIRECTION OF ROLLING AND WHICH ARE HELD AT ELEVATED TEMPERATURE IN A SUBSTANTIALLY NON-OXIDIZING ATMOSPHERE TO LOWER THE SULFUR CONTENT OF SAID STEEL, THE IMPROVEMENT ION COMBINATION THEREWITH OF ACCELERATING THE LOWERING OF SAID SULFUR CONTENT BY SUBSTANTIALLY UNIFORMLY COATING SAID COLD ROLLED STEEL WITH FROM ABOUT .02 OUNCE PER SQUARE FOOT TO ABOUT .07 OUNCE FROM ABOUT .02 OUNCE PER SQUARE FOOT TO ABOUT .07 OUNCE GROUP CONSISTING OF CALCIUM CARBONATE, LITHIUM CARBONATE, SODIUM NITRATE AND POTASSIUM NITRATE AND SUBJECTING SAID COATED COLD FOLLED STEEL TO A TEMPERATURE OF FROM 2000*F. TO 2250F. IN A SUBSTANTIALLY NON-OXIDIZING ATMOSPHERE FOR AN EFFECTIVE TIME OF UP TO BUT NOT INCLUDING ONE HOUR. 